During electrolysis electric energy is converted into chemical energy. This is achieved by the decomposition of a chemical compound by the action of an electric current. The solution used as electrolyte contains positively and negatively charged ions. Therefore, mainly acids, bases or salts are used as electrolytes.
In the production of halogen gases from aqueous alkali halide solution, for example, the following reaction takes place on the anode side:4NaCl→2Cl2+4Na++4e−  (1)The liberated alkali ions move to the cathode where they form akaline lye with the hydroxide ions obtained there. In addition, hydrogen is formed:4H2O+4e−→2H2+4OH−  (2)The lye obtained is removed from the sodium chloride, which is fed to the anode side, by means of a cation exchange membrane, thus achieving separation. Membranes of this kind are state-of-the-art and commercially available from various suppliers.
The standard potential generated on the anode when the above reaction takes place is +1.36 V, with the standard potential on the cathode being −0.86 V when the above reaction takes place. A cell design of this type is known from, for example, WO98/55670. From the difference between these two standard potentials results an enormous input of energy, which is required to conduct these reactions. In order to minimise this differential amount, gas diffusion electrodes (hereinafter referred to as GDEs) are used on the cathode side, by which means oxygen enters the system with the result that on the cathode the following reaction will take place instead of reaction (2):O2+2H2O+4e−→4OH−  (3)The overall reaction when using the NaCl-GDE technology is hence defined as follows:4NaCl+O2+2H2O→4NaOH+2Cl2  (4)As the standard potential of reaction (3) is +0.4 V, the NaCl-GDE technology results in a significant energy saving as compared to the conventional technology.
Gas diffusion electrodes have been used for many years in batteries, electrolysers and fuel cells. The electrochemical conversion takes place inside these electrodes at the so-called three-phase boundary only. Three-phase boundary is the term used for the area where gas, electrolyte and metallic conductor get into contact with one another. To make the GDE work effectively, the metallic conductor should at the same time be a catalyst for the desired reaction. Typical catalysts in alkaline systems are silver, nickel, manganese dioxide, carbon and platinum. To be particularly efficient, these catalysts must have a large surface area. This is achieved by finely divided or porous powders with specific surface area.
Problems in the use of such gas diffusion electrodes as disclosed in U.S. Pat. No. 4,614,575, for example, are due to the fact that the electrolyte would penetrate into these fine pore structures due to capillary action and fill them up. This effect would make the oxygen stop diffusing through the pores, thus stopping the intended reaction.
To achieve that the reaction takes place effectively at the three-phase boundary, the above problem is to be avoided by selecting adequate pressure ratios. The formation of a liquid column in a liquid at rest, as applies to the electrolyte solution, causes, for example, the hydrostatic pressure to be highest at the lower end of the column, which would enhance the phenomenon described above.
As documented in the relevant literature, this problem is solved by falling-film evaporators. Here, the lye is percolated through a porous material positioned between the membrane and the GDE, thus preventing the formation of a hydrostatic column. This is also referred to as percolation technology.
WO03/042430 suggests to use polyethylenes of high density or perfluorinated plastic materials for this porous percolating layer.
A principle of such kind is disclosed in DE102204018748, for example. Here, an electrochemical cell is described which consists of at least one anode compartment with an anode, one cathode compartment with a cathode and one ion exchange membrane arranged between the anode compartment and the cathode compartment, with the anode and/or cathode being a gas diffusion electrode, and a gap being provided between the gas diffusion electrode and the ion exchange membrane, and an electrolyte inlet being arranged at the upper end of the gap and an electrolyte outlet at the lower end of the gap as well as a gas inlet and a gas outlet, with the electrolyte inlet being connected to an electrolyte feed tank and having an overflow.
The electrolyte overflow is to ensure uniform feed across the whole width of the cell. The amount of electrolyte flowing from the feed tank into the electrolyte inlet depends on the difference in height between the liquid level of the electrolyte in the feed tank and the liquid level in the electrolyte inlet. The liquid level in the electrolyte inlet, in turn, depends on the height of the overflow which determines the volume of electrolyte dammed up in the electrolyte inlet.
If more electrolyte is supplied than can flow off via the overflow channel and the gap, the pressure of the electrolyte will increase in the channel-type electrolyte inlet at the upper end of the gap. The pressure in the electrolyte inlet can be adjusted by the height selected for the overflow channel. By increasing the pressure it is hence possible to pass a larger amount of electrolyte through the gap and the flow velocity inside the gap can be varied as required. By varying the ratios of the before-mentioned differences in height to one another it is possible to adjust the pressure in the electrolyte inlet as desired.
An electrolyser is referred to as an apparatus which is built up by a plurality of electrically contacted plate-type electrolysis cells arranged side by side in a stack, said cells having inlets and outlets for all liquids and gases supplied and produced. In other words, a plurality of single elements is connected in series, each element having electrodes that are separated from each other by a suitable membrane and fitted in a frame for holding these single elements. Electrolysers of such kind are disclosed, for example, in DE 196 41 125 A1 and DE 102 49 508 A1.
To protect the metal components, such as nickel, copper, silver and gold, of which an electrolysis cell with gas diffusion electrode is made, a polarisation can be performed during a downtime period as, for example, during start-up, shut-down, operational interruptions or failures. This is, for example, the case when an electrolysis cell is filled and heated for being put into operation. When the cell is taken out of electrolysis operation, the polarisation is likewise to be maintained until the anodic liquid is free of chlorine and has cooled down.
The polarisation current ensures that the metal components of the electrolysis cell are within a potential range which does not allow any corrosion reactions causing the dissolution of the metals of which the individual components of the cell cathode are made. The intensity of the polarisation current is to be selected so high that, after losses due to stray currents resulting from electrolyte feed and discharge operations, a sufficiently positive current intensity is still available in the centre of the electrolyser to ensure a defined potential range which does not allow any critical corrosion reactions.
The following shall deal with an electrochemical cell for the conventional hydrogen-producing chlor-alkali electrolysis arranged according to the state of the art disclosed in DE 196 41 125 A1 and DE 102 49 508 A1. To ensure that electrolysis cells of such kind function properly, a minimum polarisation current is to be maintained when the main electrolysis current has been turned off in order to protect the electrode coating from corrosion reactions. The way to achieve an adequate corrosion protection by polarisation currents which are as low as possible by means of a discharge channel in connection with a PTFE discharge tube is described in DE 102 49 508 A1. Here, that part of the fed polarisation current that is discharged via the electrolytes in the feed and discharge lines of the cell is minimised by the constructional measures mentioned. The feed of brine and lye is implemented via a conventional inlet manifold.
To quantify these currents, an electrolyser 1, as shown in FIG. 1A, shall be dealt with as an example in the following, the electrolyser consisting of 160 single electrolyser elements which are arranged in two electrolyser stacks 2 and 3. This electrolyser is fed with a polarisation current of 27 A on the anode side so that, without losses by stray currents, an overall voltage of theoretically approximately 250V is reached. By means of an electric model which includes the different ohmic resistances of the element components and the electrolytes as well as the respective electrochemical equations it is possible to calculate the course of the current intensity of each element. The results are shown in FIG. 1B which depicts the current of the element in relation to the element number, i.e. the position in the electrolyser.
It shows that only approx. 40% of the current reaches the elements, the other 60% are lost via stray currents. FIG. 1C and FIG. 1D give a detailed representation of the stray currents, which are conducted via the electrolyte feed and discharge flows of each element. FIG. 1C depicts stray currents in relation to the element number, i.e. the element position in the electrolyser, the stray currents being carried off via the brine feed lines (represented by unfilled triangles) and the lye feed lines (represented by filled triangles). FIG. 1D, in comparison, gives a detailed representation of the flows that are lost via the lye discharge line (shown by filled triangles) and the anolyte discharge line (shown by unfilled triangles). The disadvantage of this technology is hence that very large stray currents are produced which, in turn, require high polarisation currents.